Photoimageable compositions useful as resists for forming printed circuits, printing plates, solder masks or the like have been used for some time now. The earliest photoresists were solvent-borne and solvent-developable. The development of aqueous-developable resists represented an advance in reducing solvent emissions in the immediate workplace and in the general environment. The continued emphasis in reducing organic solvent both from the workplace and from the general environment has spurred the search for waterborne photoresists which are formulated and applied as aqueous liquids.
U.S. Pat. No. 5,045,435, the teachings of which are incorporated herein by reference, describes a waterborne photoresist composition which is developable in alkaline aqueous solution. The composition comprises a multifunctional monomer, a photoinitiator and a latex of water-insoluble carboxylated acrylic copolymer. To stabilize the composition and to adjust the viscosity, this patent teaches neutralizing the latex polymer to at least 25% with a base, such as ammonia, another amine, or sodium hydroxide.
In photoimageable compositions, such as described in U.S. Pat. No. 5,045,435, where ammonia, primary, secondary or tertiary amines are used to neutralize the carboxylic acid functionality of the binder, it is attempted to remove the amines during the drying process. Any residual base creates tremendous water sensitivity. As a result, the drying window using these materials is narrow. In addition, the desired volatility of the amines create potential environmental concerns and compromise the fundamental reason for waterborne coatings in the first place, i.e., non-hazardous, emission-free coatings. Moreover, when "free amines" are used for the neutralizations, strong chelate bonds can also form with the copper surface, resulting in reactivity problems developing residues and, subsequently, etch retardation.